Hydraulic motors are used for applications requiring plenty of torque, performance, constant reversals of rotary drive directions, or a compact size. Hydraulic motors can also be used when conditions are difficult; such as humidity, dustiness, or a high temperature. In mobile equipment, hydraulic drive has almost completely superseded other drives by virtue of these benefits.
Until now, it has been necessary to provide heavy-duty hydraulic motors with three or four hydraulic lines. Pressure and return lines are always included, but often the system comprises also a so-called drain line, whereby the hydraulic fluid draining into a motor casing is returned to the tank and recirculation. Larger motors, in particular, are always provided with a drain line. The pressure of oil draining into a casing would rise at least to equal the pressure of a return line if there was no drain line. In practice, such a pressure is not acceptable. Four lines are required in the system if a separate cool-down flushing circulation is provided for the casing.
Many hydraulically operated systems, such as bucket machines, employ primarily hydraulic cylinders to work. Hydraulic cylinders do not require a drain connection and, thus, the hydraulic piping of bucket machines does not include a drain oil line as a standard feature and, therefore, it must be separately installed for a hydraulic motor included in an accessory, for example. It is also often the case that a hydraulic motor must be installed far away from the actual pump or tank, resulting in a long drain line. Especially in equipment, operating deep underwater or in mines, the extra line causes problems and more expenses. If the drain oil connection could be omitted, the coupling of a motor-equipped actuator with any hydraulic system would be simpler.
In order to enable the oil seeped into a casing to proceed to main lines, the pressure level of such oil should be raised to be equal to or higher than the pressure of a receiving line, without increasing pressure in the casing. This elevation of pressure can be performed with a pump. A problem here is driving power for the pump, since the number of hydraulic links must not increase. If the energy is picked up directly from the oil stream and pressure difference between pressure and return lines, the system requires in practice at least a hydraulic motor and a pump. Reversal of the rotating direction must also be taken into account in the system configuration. In order to make the system as simple as possible, the extra motor is not worth installing but, instead, it is reasonable to implement this type of solution by using the method disclosed in the Applicant's patent application WO 01/65113, wherein driving power for the pump is taken directly from the shaft of a main motor.
In an effort to further simplify the design and in search of alternative sources of driving power, it has been discovered in the invention to utilize pressure differences existing in the system. Outside the divider of a motor, the pressure in a working line, as the motor is running, is always higher than in a return line, and the pressure difference does not fluctuate if the loading does not fluctuate. In practice, this denies the use of a simple pump for the removal of drain oil in a solution effected outside the divider.
The invented solution involved the use of intra-motor pressure differences. Hydraulic motor must always have an element which opens flow channels for oil flowing in and out of the motor in order to enable the actuators, such as pistons, to set the output shaft in rotation. This element, which is referred to as a divider, may comprise for example a rotating wheel provided with channels for guiding the flow of hydraulic fluid in and out of intra-motor channels, or a valve type solution capable of corresponding actions. Thus, the intra-divider oil channels or channel are or is pressurized in pulses according to rotation. Since one and the same channel functions alternately as a working or pressure channel and alternately as a return channel, said channel experiences alternately over a single cycle both a high working pressure and a low return pressure. The magnitude of a pressure difference in the channel over a single cycle varies according to loading. It should be appreciated that this pressure pulse also develops in the channels even if the motor is under uniform loading or idling.